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Solar Still 3-D CFD Simulation Using Species Transport Model

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More than 97 percent of the earth’s water is salty water, and with the increasing population, increasing agricultural consumption and increasing water and energy consumption in industry and agriculture, the world is rapidly developing solar still industry.

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Problem Description for Desalination

The present study investigates the performance of a solar-still thermal desalination unit. The present model consists of a small chamber with a sloping glass surface at the top. The solar heat passes through the glass to the surface of the water in the enclosure and causes surface evaporation (applying a UDF for surface evaporation). The resulting vapor impact to the cold glass surface and undergo a distillation process. Pure water from hot vapor distillation moves down the slope of the glass plate and discharges as pure water.

To solve this problem, the Species Transport method is used. When using this method, saline water flow is not simulated in the device, unlike methods such as multiphase simulations that have much heavier computations. This assumes that there is a mixture of air and vapor inside the chamber interior. In fact, the simulation focuses solely on the amount of vapor produced in the system, and the purpose of the present study is to investigate the amount of vapor produced on the water surface and the cold surface of the sloping plate.

The Assumption for Solar Still CFD Simulation

There are several assumptions to simulate the present problem:

The simulation is Steady-State and the solver is Pressure-Based. The effect of the earth’s gravity on the flow is considered equal to 9.81 m.s-2 because in this problem the glass plate slope causes the downward movement of the distilled water.

Geometry and Mesh

The present 3-D model was designed by Design Modeler software. The geometry consists of a sloping glass surface at the top and a horizontal water plate at the bottom.

A structured mesh was performed using ANSYS Meshing software. The element number is 500,000.

Solar Still CFD Simulation

Here is a summary of the steps to define and solve the problem:

Laminar Viscous model
Species transport Species
air and vapor species
on Energy
Boundary conditions for desalination
wall Walls type
0 W.m-2 heat flux back, front, side walls
zero diffusive flux for h2O
343.95 K temperature ground walls
1 mass fractions for h2O
338.05 K temperature glass walls
1 mass fractions for h2O
Solution Methods for solar still simulation
Simple   Pressure-velocity coupling
PRESTO pressure Spatial discretization
first order upwind momentum
first order upwind energy
first order upwind h2O
Initialization for Desalination simulation
Standard Initialization method
341 K temperature
0 m.s-1 velocity (x, y, z)

Species Transport

This method assumes that there is a mixture of air and vapor inside the chamber interior and does not simulate distilled water flow. In fact, at the beginning of the simulation, the space inside the chamber is filled with air only; the bottom plane of the chamber is considered as the surface of the water, and since the water evaporates on the surface, it is assumed that the surface consists of vapor. It is also assumed that the sloping plate where the steam collides with, is composed of vapor only.

The equation governing the flow of gaseous species present in the simulation is as follows:


Species Transport

In the above equation, Yi represents the gas species present in the equation, which includes air and vapor in the present model. The first term on the left is equivalent to the time-dependent term (if unsteady) and the second term is the equivalent of the transport term. The first term is the equivalent of the diffusion term (including mass and thermal diffusion), the second term is the equivalent of the chemical reactions term (if any) and the third term is for heat sources (if any).

The two mass diffusivity and thermal diffusion coefficient parameters indicate mass diffusion and thermal diffusion, respectively. such that the mass diffusion coefficient is equal to the constant value of 2.88⨯10-5 m2.s-1 and the thermal diffusion coefficient is equivalent to the conductivity coefficient to specific heat capacity ratio, which is equal to 4.56754⨯10-5 kg.m-1.s-1.

There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.


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